1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical transmission system for transmitting wavelength division multiplexed (WDM) signal light, as well as to an optical amplifier and a dispersion compensator both applicable to such a system; and more particularly to a wavelength division multiplexing optical transmission system, optical amplifier and dispersion compensator adapted to utilize a hybrid transmission path comprising combined optical fibers having wavelength dispersion characteristics conflicting with each other, so as to effectively compensate wavelength dispersions of respective wavelength bands, in case of transmitting broadband wavelength division multiplexed signal light containing a plurality of wavelength bands.
2. Related Art
Conventionally, there has been performed transmission of an optical signal, by utilizing an optical regenerating repeater which converts an optical signal into an electric signal to thereby perform retiming, reshaping and regenerating. At present, however, practical use of an optical amplifier has been advanced, so that there is being investigated an optical amplifying-and-repeating transmission method which adopts an optical amplifier as a linear repeater. By substituting an optical regenerating repeater with an optical amplifying repeater, it is expected that the number of parts within the repeater is remarkably reduced to thereby ensure reliability and permit cost reduction. Further, as one method for realizing a large capacity of an optical transmission system, attention has been directed to a wavelength division multiplexing (WDM) optical transmission method which multiplexes two or more optical signals having wavelengths different from each other to transmit within a transmission path.
In a WDM optical amplifying-and-repeating transmission method obtained by combining the aforementioned optical amplifying-and-repeating transmission method and WDM optical transmission method, it is possible to collectively amplify WDM signal lights making use of an optical amplifier, to thereby permit realization of large capacity and long distance transmission with a simple (economic) constitution.
In the conventional WDM optical amplifying-and-repeating transmission system (hereinafter abbreviated to xe2x80x9cWDM optical transmission systemxe2x80x9d), there is used a method for managing a wavelength dispersion of a transmission path, so as to reduce transmission characteristic degradation due to non-linear effect of a transmission path.
For example, in an article xe2x80x9cWavelength Division Multiplexing in Long-Haul Transmission Systems, IEEE Journal of Lightwave Technology. vol. 14, No. 6, pp. 1299-1308, 1996xe2x80x9d of N. S. Bergano et al., there is used a transmission path obtained by combining: a dispersion-shifted fiber (DSF) of a length of about 900 km having a zero-dispersion wavelength of 1585 nm and a positive wavelength dispersion slope; with a single mode fiber (SMF) of a length of about 100km having a zero-dispersion wavelength of 1310 nm and a positive wavelength dispersion slope. This transmission path has an averaged zero-dispersion wavelength of about 1558 nm, and accommodates wavelengths of signal lights ranging from 1556 nm to 1560 nm.
Wavelength dispersions of DSF and SMF are approximately xe2x88x922 ps/nm/km and +20 ps/nm/km, respectively, in which a group velocity of signal light and spontaneous emission light and a group velocity of mutual signal lights are different from each other. Thus, by adopting a transmission path obtained by combining DSF and SMF, it becomes possible to shorten an interaction period of time of non-linear effect, and to reduce degradation of a transmission characteristic such as due to four wave mixing (FWM) and a cross phase modulation (XPM). Further, since the transmission path has the averaged zero-dispersion wavelength within a signal light wavelength, there is also reduced a degradation of transmission characteristic due to self phase modulation (SPM) and wavelength dispersion.
However, when it is required to expand a transmission band to thereby increase a capacity of a WDM optical transmission system, it will be difficult to compensate such that wavelength dispersions become zero for all signal light wavelengths in the above described constitution, because of an affection of wavelength dispersion slope. As such, there are necessarily caused degradations of signal light waveforms due to interaction between: wavelength dispersion which is not compensated but accumulated; and non-linear effect within an optical fiber.
As a countermeasure for such a situation, there has been proposed a transmission path having a latter stage of a transmission section thereof applied with a dispersion compensation fiber for compensating wavelength dispersions and dispersion slopes thereof caused in a former stage of the transmission section. Concretely, it has been proposed to reduce dispersion slope to thereby reduce accumulated wavelength dispersions so as to reduce a degradation of a transmission characteristic, by adopting: for a former stage of a transmission section, a 1.3 xcexcm zero-dispersion SMF having a positive wavelength dispersion and a positive dispersion slope; and for a latter stage of the transmission section, a dispersion compensation fiber having a negative wavelength dispersion and a negative dispersion slope, which compensates the wavelength dispersion and dispersion slope of the 1.3 xcexcm zero-dispersion fiber.
In an article xe2x80x9cQuarter terabit (25xc3x9710 Gb/s) over 9288 km WDM transmission experiment using nonlinear supported RZ pulse in higher order fiber dispersion managed line, ECOC ""98, pp. 79-81, 1998xe2x80x9d of M. Murakami et al., it has been permitted to reduce an averaged wavelength dispersion slope up to 0.0067 ps/nm2/km, by adopting: for a former section of a transmission section, a 1.3 xcexcm zero-dispersion fiber of a 50% length of the transmission section having a positive wavelength dispersion; and for a latter stage of the transmission section, a dispersion compensation fiber of a 50% length of the transmission section having a negative wavelength dispersion.
There has been recently further proposed an optical transmission technique adopting WDM signal light including a plurality of wavelength bands such as 1550 nm band and 1580 nm, so as to increase a transmission capacity of a WDM optical transmission system.
According to, for example, an article xe2x80x9cUltra-wide band, long distance WDM transmission demonstration: 1 Tb/s (50xc3x9720 Gb/s), 600km transmission using 1550 and 1580 nm wavelength bands, PD11, OFC ""98, 1998xe2x80x9d of S. Aisawa, it has been permitted to increase a transmission capacity by adopting WDM signal lights of two wavelength bands, 1550 nm band and 1580 nm band, to thereby allow 50 waves of signal lights to be multiplexed. Here, since the wavelength dispersions and the dispersion slopes accumulated within SMF transmission path are different from each other for the respective wavelength bands, respectively, there are inserted dispersion compensators for compensating wavelength dispersions and dispersion slopes in the respective wavelength bands, into optical amplifiers linked in multiple stages, respectively. Each of the optical amplifiers has a constitution to demultiplex the input WDM signal lights in each of wavelength bands, and to send the demultiplexed WDM signal lights of each of the wavelength bands to each of corresponding dispersion compensators so as to compensate wavelength dispersion and dispersion slope caused within the SMF transmission path for each of wavelength bands. In this way, there is reduced a degradation of transmission characteristic of WDM signal lights in both wavelength bands of 1550 nm band and 1580 nm band.
Further, there has been proposed a technique for transmitting WDM signal light of a plurality of wavelength bands, making use of a hybrid transmission path obtained by combining a plurality of optical fibers having wavelength dispersion characteristics different from each other such as described above.
For example, in an article xe2x80x9c765 Gb/s over 2,000 km Transmission Using C- and L-band Erbium Doped Fiber Amplifiers, PD16-1, OFC ""99, 1999xe2x80x9d of Mattehw X. Ma et al., there has been disclosed a technique for repeatingly transmitting WDM signal lights of 1550 nm band and 1580 nm band, making use of a hybrid transmission path obtained by combining a normal optical fiber (SMF), a dispersion-shifted fiber (NZ-DF) and a dispersion compensation fiber (DCF). In this technique, there are used, as transmission path, three types of optical fibers having wavelength dispersion characteristics different from one another, so as to reduce an averaged dispersion slope over two wavelength bands, and so as to perform dispersion compensation of demultiplexed WDM signal lights for each of the wavelength bands, to thereby realize a long distance transmission of WDM signal lights of 1550 nm band and 1580 nm band.
However, in a conventional WDM optical transmission system for transmitting WDM signal light having a plurality of wavelength bands as described above, it becomes necessary to compensate the accumulated wavelength dispersion and the dispersion slope for each of wavelength bands for a broadband WDM signal light. As such, there have been problematically complicated such as optical amplifiers provided with dispersion compensators corresponding to respective wavelength bands, thereby leading to higher cost.
Further, wavelength dispersion accumulated in a transmission path adopting a normal SMF has a positive value for a wavelength band such as 1550 nm band and 1580 nm band. As such, dispersion compensation fiber such as provided in an optical amplifier for each of wavelength bands is required to have a negative wavelength dispersion. However, because a modular dispersion compensation fiber, which has such a negative wavelength dispersion and is to be provided within an optical amplifier, has a mode field diameter smaller than that of a normal SMF, there is such a defect that: the broader the band of WDM signal light is, the more susceptively the WDM signal light is affected by non-linear effect.
In the above described conventional method for compensating wavelength dispersion, even when a hybrid transmission path is adopted to thereby reduce accumulation of wavelength dispersion and dispersion slope as described above, the compensation by the hybrid transmission path only is insufficient for a whole of broad wavelength band of WDM signal light, and it has been required to compensate wavelength dispersion for each of wavelength bands.
There will be now considered a system described in the above article of Matthew X. Ma et al.
As shown in a wavelength dispersion map of FIG. 34, upon calculating wavelength dispersions in one section (43.5 km) of a hybrid transmission path based on the description of the article, it is considered that there will be accumulated a positive wavelength dispersion in the order of about 7 ps/nm for a shortest wavelength (1529.6 nm) and in the order of about 50 ps/nm for a longest wavelength (1600 nm). Further, the FIG. 1 of the article shows a wavelength dispersion map of a whole transmission section (2000 km) for a 1550 nm band, which map exhibits that there are accumulated a positive wavelength dispersion in the order of about 500 ps/nm in the channel 1 (shortest wavelength of 1550 nm band) and about 2,000 ps/nm in the channel 50 (longest wavelength of 1550 nm band). In this way, there are accumulated positive wavelength dispersions for respective wavelength bands, even when a hybrid transmission path is adopted so as to compensate wavelength dispersion and dispersion slope. This appears to be why dispersion compensators having negative wavelength dispersions are provided in an optical amplifier corresponding to respective wavelength bands, so as to compensate the accumulated wavelength dispersions to be zero. There is thus caused such a problem in this case that the optical amplifier is complicated to lead to higher cost, and is susceptible to a non-linear effect.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide a WDM optical transmission system for effectively compensating such as wavelength dispersions of respective wavelength bands by a simple constitution making use of a hybrid transmission path in case of transmitting broadband WDM signal light containing a plurality of wavelength bands, to thereby realize excellent transmission characteristics.
To achieve the above object, according to one aspect of the present invention, a wavelength division multiplexing optical transmission system for transmitting wavelength division multiplexed signal light including a plurality of wavelength bands comprises: an optical transmission path comprising a first transmission section provided by connecting: a first optical fiber having positive wavelength dispersions and positive dispersion slopes relative to the plurality of wavelength bands, respectively; and a second optical fiber having negative wavelength dispersions and negative dispersion slopes relative to the plurality of wavelength bands, respectively; the first transmission section having wavelength dispersion characteristics set such that compensation ratios by the second optical fiber for wavelength dispersions and dispersion slopes caused within the first optical fiber become maximum for a reference wavelength band which is one of the plurality of wavelength bands; and a dispersion compensation device capable of compensating wavelength dispersions caused within the first transmission section of the optical transmission path, for the plurality of wavelength bands except for the reference wavelength band.
According to the WDM optical transmission system having such a constitution, the WDM signal light including the plurality of wavelength bands is transmitted through the first transmission section of the optical transmission path, so that wavelength dispersions and dispersion slopes caused within the first optical fiber are compensated by the second optical fiber. The compensation ratios are set to be maximum for the reference wavelength band to thereby fully perform wavelength dispersion compensation for the reference wavelength band. However, compensation errors for those wavelength bands except for the reference wavelength band become large, so that wavelength dispersions and dispersion slopes will remain. Nonetheless, the residues of at least wavelength dispersions (and of dispersion slopes, depending on setting of the first transmission section) are compensated by the dispersion compensation device for each of the wavelength bands except for the reference wavelength band. In this way, dispersion compensations for the plurality of wavelength bands can be assuredly performed by a simple constitution, to thereby allow cost reduction in a WDM optical transmission system.
According to another aspect of the present invention, a wavelength division multiplexing optical transmission system for transmitting wavelength division multiplexed signal light including a plurality of wavelength bands comprises: an optical transmission path comprising a first transmission section provided by connecting: a first optical fiber having positive wavelength dispersions and positive dispersion slopes relative to the plurality of wavelength bands, respectively; and a second optical fiber having negative wavelength dispersions and negative dispersion slopes relative to the plurality of wavelength bands, respectively; the first transmission section having wavelength dispersion characteristics set such that compensation ratios by the second optical fiber for wavelength dispersions caused within the first optical fiber become approximately 100% for a central wavelength of a shortest wavelength band and for a central wavelength of a longest wavelength band of the plurality of wavelength bands, respectively. The wavelength division multiplexing optical transmission system may further comprise a dispersion compensation device for compensating wavelength dispersions caused within the first transmission section of the optical transmission path, for intermediate wavelength bands positioned between the shortest wavelength band and the longest wavelength band.
According to the WDM optical transmission system having such a constitution, the compensation ratios within the first transmission section of the optical transmission path become approximately 100% for a central wavelength of a shortest wavelength band and for a central wavelength of a longest wavelength band of the plurality of wavelength bands, respectively. Thus, wavelength dispersions for the central wavelengths become approximately zero. At this time, for dispersion slopes, approximately 100% of compensation is realized near a wavelength band intermediate between the shortest wavelength band and the longest wavelength band, but slight compensation errors are caused at the shortest wavelength band and the longest wavelength band. Nonetheless, these compensation errors of dispersion slopes can be regarded as being such degrees that have no affection on transmission characteristics, when considering band widths of respective wavelength bands. Thus, even without any dispersion compensations for the respective wavelength bands, there can be obtained satisfactory transmission characteristics for the plurality of wavelength bands. For the intermediate wavelength bands, there is caused such a possibility that wavelength dispersion compensations within the first transmission section become insufficient. Nonetheless, in such a situation, wavelength dispersion compensations for the plurality of wavelength bands can be realized by a simple constitution, by providing the dispersion compensation device which performs wavelength dispersion compensations for the intermediate wavelength bands only. In this way, it becomes possible to achieve cost reduction in a WDM optical transmission system.
According to yet another aspect of the present invention, a wavelength division multiplexing optical transmission system for transmitting wavelength division multiplexed signal light including a plurality of wavelength bands comprises: an optical transmission path comprising a first transmission section provided by connecting: a first optical fiber having positive wavelength dispersions and positive dispersion slopes relative to the plurality of wavelength bands, respectively; and a second optical fiber having negative wavelength dispersions and negative dispersion slopes relative to the plurality of wavelength bands, respectively; the first transmission section having wavelength dispersion characteristics set such that compensation ratios by the second optical fiber for wavelength dispersions caused within the first optical fiber become maximum for a reference wavelength band which is one of the plurality of wavelength bands; and a dispersion compensation device capable of compensating wavelength dispersions caused within the first transmission section of the optical transmission path, for the plurality of wavelength bands, respectively, except for the reference wavelength band.
According to the WDM optical transmission system having such a constitution, the compensation ratios only for wavelength dispersions within the first transmission section of the optical transmission path are set to become maximum for the reference wavelength band. Thus, compensation errors become larger for the wavelength bands except for the reference wavelength band, so that wavelength dispersions and dispersion slopes will remain. Nonetheless, the residues of at least wavelength dispersions (and of dispersion slopes, depending on setting of the first transmission section) are compensated by the dispersion compensation device for each of the wavelength bands except for the reference wavelength band.
According to still another aspect of the present invention, a wavelength division multiplexing optical transmission system comprises: an optical transmission path through which wavelength division multiplexed signal light including a plurality of wavelengths for communication is transmitted, the optical transmission path comprising: a first optical fiber having positive wavelength dispersions and positive dispersion slopes relative to the plurality of wavelength, respectively; and a second optical fiber having negative wavelength dispersions and negative dispersion slopes relative to the plurality of wavelength, respectively; the first optical fiber and the second optical fiber having wavelength dispersion characteristics set such that compensation ratios by the second optical fiber for wavelength dispersions and dispersion slopes caused within the first optical fiber become maximum for a reference wavelength which is one of the plurality of wavelengths; and a dispersion compensation device capable of compensating wavelength dispersions caused within the optical transmission path, for each of predetermined wavelength widths for the plurality of wavelengths, respectively.
According to the WDM optical transmission system having such a constitution, there can be compensated such as wavelength dispersions caused within the optical transmission path, by treating each of the predetermined wavelength widths as one unit, for the plurality of wavelengths for communication.
The present invention further provides an optical amplifier for amplifying wavelength division multiplexed signal light, the optical amplifier comprising: a demultiplexing part for separating wavelength division multiplexed signal lights from an optical transmission path, corresponding to respective wavelengths; optical amplifying parts for amplifying the lights of the respective wavelengths, separated by the demultiplexing part; dispersion compensation parts for performing dispersion compensations corresponding to wavelength dispersion values of the lights of the respective wavelengths, separated by the demultiplexing part; and a multiplexing part for wavelength-division multiplexing the lights of the respective wavelengths, which lights have been treated by the optical amplifying part and the dispersion compensation parts, respectively.
According to the optical amplifier having such a constitution, wavelength division multiplexed signal lights sent from the optical transmission path are demultiplexed by the demultiplexing part corresponding to the respective wavelengths, and the demultiplexed lights are sent to the optical amplifying parts and dispersion compensation parts. The demultiplexed lights of the respective wavelengths have wavelength dispersions corresponding to characteristics of the optical transmission path through which the lights have been propagated, respectively, and the dispersion compensation parts perform dispersion compensations corresponding to the wavelength dispersion values, respectively. The lights, which have been amplified by the optical amplifying parts and dispersions of which have been compensated by the dispersion compensation parts, are multiplexed by the multiplexing part into wavelength division multiplexed signal light.
The present invention also provides a dispersion compensator provided within an optical transmission path for transmitting wavelength division multiplexed signal lights, the compensator comprising: a demultiplexing part for separating wavelength division multiplexed signal lights from the optical transmission path, corresponding to respective wavelengths; and dispersion compensation parts for performing dispersion compensations corresponding to wavelength dispersion values of the lights of the respective wavelengths, separated by the demultiplexing part.
According to the dispersion compensator having such a constitution, wavelength division multiplexed signal lights from the optical transmission path are demultiplexed by the demultiplexing part corresponding to the respective wavelengths. The demultiplexed lights of the respective wavelengths have wavelength dispersions corresponding to characteristics of the optical transmission path through which the lights have been propagated, respectively, and the dispersion compensation parts perform dispersion compensations corresponding to the wavelength dispersion values, respectively.